The present invention relates to a method of inspecting a reticle or a photomask (hereinafter referred to inclusively as a "reticle"), provided with a circuit pattern and a phase shifter formed of a light-transmissive film for defects, such as foreign particles adhering to the reticle; and, more particularly, to a method of inspecting a reticle provided with a phase shifter for defects, such as foreign particles, having sizes in the submicron range before printing the reticle on a wafer, and a reticle inspecting apparatus for carrying out the same reticle inspecting method.
When fabricating LSI chips or printed wiring boards, a reticle having a circuit pattern is inspected for defects before printing the reticle on wafers by a photographic process. If the reticle has minute foreign particles having sizes in the submicron range thereon, the reticle cannot be correctly printed on wafers; and, consequently, LSI chips fabricated by using such wafers become defective. Problems attributable to the minute foreign particles adhering to the reticle have become more and more significant with the recent progressive increase in the degree of integration of LSIs, and the existence of foreign particles even having sizes in the submicron range on the reticle is not permissible.
The inspection of the reticle for foreign particles before printing the reticle on a wafer is indispensable to prevent defective reticle printing, and various techniques for inspecting the reticle for foreign particles have been proposed. A prevalent method of inspecting a reticle for foreign particles, which is employed widely because of its capability of quick and highly sensitive inspection, irradiates the reticle obliquely with a light beam having a high directivity, such as a laser beam, and detects light scattered by foreign particles. However, since the light beam is diffracted at the edges of the pattern of the reticle, the diffracted light and the light scattered by foreign particles must be discriminated from each other. Various technical means for discriminating between the diffracted light and the scattered light have been proposed.
A first previously proposed technical approach is exemplified by an inspecting apparatus disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 54-101390. This inspecting apparatus comprises a laser that emits a linearly polarized laser beam, an irradiating means for irradiating a circuit pattern obliquely with the linearly polarized laser beam so that the linearly polarized laser beam falls on the circuit pattern at a given incidence angle, and an oblique focusing optical system including a polarizing plate and lenses. When the circuit pattern is irradiated with the linearly polarized laser beam, the light diffracted by the circuit pattern and the light scattered by foreign particles differ from each other in the plane of polarization, i.e., the plane of vibration; therefore, the light scattered by the foreign particles can be discriminated and detected.
A second previously proposed technical approach is exemplified by an inspecting apparatus disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 59-65428, 1-117024 or 1-153943. This inspecting apparatus comprises a scanning arrangment for scanning an object with a laser beam obliquely projected on the object, a first lens disposed above the object to condense scattered laser light so that the point of irradiation of the laser beam coincides substantially with the condensing point, a filter plate disposed on the Fourier transform image plane of the first lens to filter regular diffracted light diffracted by the circuit pattern of the object, a second lens for the inverse Fourier transform of the light scattered by the foreign particles and transmitted through the screen plate, a slit plate disposed at the focal point of the second lens to screen scattered light from portions of the object other than a portion corresponding to the point of irradiation with the laser beam, and a light receiving device for receiving the light scattered by the foreign particles and which passes through the slit of the slit plate. This inspecting apparatus operates on the basis that, generally, the elements of a circuit pattern are extended in a single direction or in several directions, so that the diffracted light diffracted by the elements of the circuit pattern extending in a specified direction can be filtered by a spatial filter disposed on the Fourier transform image plane, making it possible to detect only the light scattered by the foreign particles.
A third previously proposed technical approach is exemplified by an arrangement disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 58-62543. This arrangement operates on the basis that light diffracted by the edges of a circuit pattern is directional light, while light scattered by foreign particles is not directional and identifies foreign particles on the basis of the logical product of the outputs of a plurality of obliquely arranged detectors.
A fourth previously proposed technical approach is exemplified by an arrangement disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 60-154634 or 60-154635. This arrangement is based on the fact that light diffracted by the edges of a circuit pattern converges only along a specific direction, while light scattered by foreign particles is scattered in all directions and identifies foreign particles from the outputs of a plurality of detectors.
Apparatuses and methods relating to the inspection of objects for minute foreign particles, such as a schlieren method, a phase-contrast microscope and a technique relating to a diffraction image having a finite size are disclosed in, for example, Hiroshi Kubota, "Oyou Kogaku", Iwanami Zensho, pp. 129-136.
In an array type detector, such as a one-dimensional solid-state imaging device provided with an array of solid-state image sensors, an output signal representing a foreign particle is distributed to a plurality of pixels if the foreign particle corresponds to a plurality of elements of the detector; and, consequently, the output of the detector is reduced, and therefore, there is the possibility that the detector fails in detecting the foreign particle. An invention made to obviate such a possibility, disclosed in Japanese Patent Laid-open (Kokai) No. 61-104242, disposes an array type detector at an angle to the direction of the scanning of an inspection table. Other inventions made for the same purpose, disclosed in Japanese Patent Laid-open (Kokai) Nos. 61-104244 and 61104659, employ an array type detector having a unique shape and provided with elements arranged in a unique arrangement.
Irregular illumination and the variation of illumination adversely affects the repeatability and accuracy of detection. An invention disclosed in Japanese Patent Laid-open (Kokai) No. 60-038827 calibrates the intensity of scattered light automatically by using a standard sample having known characteristics.
Japanese Patent laid-open (Kokai) No. 56-132549 discloses an invention for obviating the misidentifying of a large amount of light scattered by a comparatively large foreign particle as light scattered by a plurality of comparatively small foreign particles.
As mentioned above, failure in finding foreign particles which adversely affect the quality of LSI chips has become a significant problem with the reduction of the size of foreign particles to be detected. The first previously proposed technical approach, for example, the invention disclosed in Japanese Patent Laid-open (Kokai) No. 54-101390, is unable to detect minute foreign particles because the difference between the plane of polarization of the light scattered by minute foreign particles and the plane of polarization of the light diffracted by the edges of the circuit pattern is small.
The second previously proposed technical approach, for example, the inventions disclosed in Japanese Patent Laid-open (Kokai) Nos. 59-65428, 1-117024 and 1153943, detect only the light scattered by foreign particles by separating the light scattered by foreign particles from the light diffracted by the circuit pattern with the filter plate and the slit plate. Although these inventions use a detecting mechanism for detecting foreign particles by a simple binary method having a simple configuration to their advantage, the light diffracted by the intersection of the elements of the circuit pattern does not travel unidirectionally like the light diffracted by the straight edge of the circuit pattern, and hence, the spatial filter is unable to filter the light diffracted by the intersection of the elements of the circuit pattern perfectly. Furthermore, since the light diffracted by a minute circuit pattern having a size in the micron range for a LSI having a very high degree of integration is analogous in behavior to the light scattered by foreign particles, it is practically difficult to discriminate between the circuit pattern and foreign particles by a simple binary method.
The apparatus proposed as the third previously proposed technical arrangement in, for example, Japanese Patent Laid-open (Kokai) No. 58-62543 and those proposed as the fourth previously proposed technical arrangement in, for example, Japanese Patent Laid-open (Kokai) Nos. 60-154634 and 60-154635 have difficulty in employing an optical system having a sufficiently high condensing ability because of their configurations; and hence, it is practically difficult for these apparatuses to detect faint light scattered by foreign particles.
The apparatuses disclosed as the fifth previously proposed technical arrangement in, for example, Japanese Patent Laid-open (Kokai) Nos. 61-104242 and 61-104244 requires a special detector and a special optical system, which are costly.
The apparatus disclosed as the sixth previously proposed technical arrangement in, for example, Japanese Patent Laid-open (Kokai) No. 60-038827 has drawbacks in application to an array type detector suitable for quick detection and in structural accuracy for detecting minute foreign particles.
The apparatus disclosed as the seventh previously proposed technical arrangement in, for example, Japanese Patent Laid-open (Kokai) No. 56-132544 detects only a single point on a large foreign particle; and hence, the apparatus is unable to recognize the shape of an elongate foreign particle accurately.
A reticle recently developed to improve resolution in transferring a circuit pattern formed on the reticle is provided with a transparent or translucent thin film, which is called a phase shift film or a phase shifter, having a thickness equal to an odd number of times half the wavelength of the light used for exposure and formed so as to cover spaces between the elements of the circuit pattern. Although this thin film is transparent or translucent, the thickness of this thin film is several times the thickness (on the order of 0.1 .mu.m) of the circuit pattern. Consequently, the intensity of the light diffracted by the edges of the thin film is several to several tens of times the intensity of the light diffracted by the edges of the circuit pattern, thus reducing the foreign particle detecting sensitivity significantly.